## Timelike Showers

The PYTHIA algorithm for timelike final-state showers is based on the article Sjo05, where a transverse-momentum-ordered evolution scheme is introduced, with the extension to fully interleaved evolution covered in Cor10a. This algorithm is influenced by the previous mass-ordered algorithm in PYTHIA Ben87 and by the dipole-emission formulation in Ariadne Gus86. From the mass-ordered algorithm it inherits a merging procedure for first-order gluon-emission matrix elements in essentially all two-body decays in the standard model and its minimal supersymmetric extension Nor01.

The normal user is not expected to call `TimeShower` directly, but only have it called from `Pythia`. Nonetheless, some of the parameters below, in particular `TimeShower:alphaSvalue`, would be of interest for uncertainty estimates and tuning exercises. Note that PYTHIA also incorporates an automated framework for shower uncertainty variations.

### Main variables

Often the maximum scale of the FSR shower evolution is understood from the context. For instance, in a resonance decay half the resonance mass sets an absolute upper limit. For a hard process in a hadronic collision the choice is not as unique. Here the factorization scale has been chosen as the maximum evolution scale. This would be the pT for a 2 → 2 process, supplemented by mass terms for massive outgoing particles. For some special applications we do allow an alternative.

Way in which the maximum shower evolution scale is set to match the scale of the hard process itself. `(i) if the final state of the hard process (not counting subsequent resonance decays) contains at least one quark (u, d, s, c ,b), gluon or photon then pT_max is chosen to be the factorization scale for internal processes and the `scale` value for Les Houches input; (ii) if not, emissions are allowed to go all the way up to the kinematical limit (i.e. to half the dipole mass). This option agrees with the corresponding one for spacelike showers. There the reasoning is that in the former set of processes the ISR emission of yet another quark, gluon or photon could lead to double-counting, while no such danger exists in the latter case. The argument is less compelling for timelike showers, but could be a reasonable starting point.` `always use the factorization scale for an internal process and the `scale` value for Les Houches input, i.e. the lower value. This should avoid double-counting, but may leave out some emissions that ought to have been simulated. (Also known as wimpy showers.)` `always allow emissions up to the kinematical limit (i.e. to half the dipole mass). This will simulate all possible event topologies, but may lead to double-counting. (Also known as power showers.)`

Note 1:

as enumerated in the text, these options take effect both for internal and external processes. Whether a particular option makes sense depends on the context. For instance, if events for the same basic process to different orders are to be matched, then option 1 would be a reasonable first guess. But in more sophisticated descriptions option 2 could be combined with `UserHooks` vetoes on emissions that would lead to double-counting, using more flexible phase space boundaries. Further details are found in the Matching and Merging description, with an example in `examples/main31`. Option 0, finally, may be most realistic when only Born-level processes are involved, possibly in combination with a nonzero `TimeShower:pTdampMatch`.

Note 2:

These options only apply to the hard interaction. If a "second hard" process is present, the two are analyzed and set separately for the default 0 option, while both are affected the same way for non-default options 1 and 2. Emissions off subsequent multiparton interactions are always constrained to be below the factorization scale of each process itself. The options also assume that you use interleaved evolution, so that FSR is in direct competition with ISR for the hardest emission. If you already generated a number of ISR partons at low pT, it would not make sense to have a later FSR shower up to the kinematical limit for all of them.

Note 3:

Recall that resonance decays are not affected by this mode, but that showers there are always set to fill the full phase space, often with built-in matrix-element-matching that give a NLO accuracy. A modification of this behaviour would require you to work with `UserHooks`. However, for Les Houches input the optional `Beams:strictLHEFscale = on` setting restricts all emissions, also in resonance decays, to be below the input `scale` value.
In cases where the above `pTmaxMatch` rules would imply that pT_max = pT_factorization, `pTmaxFudge` introduces a multiplicative factor f such that instead pT_max = f * pT_factorization. Only applies to the hardest interaction in an event, and a "second hard" if there is such a one, cf. below. It is strongly suggested that f = 1, but variations around this default can be useful to test this assumption.

Note:

Scales for resonance decays are not affected, but can be set separately by user hooks.
A multiplicative factor f such that pT_max = f * pT_factorization, as above, but here for the non-hardest interactions (when multiparton interactions are allowed).
These options only take effect when a process is allowed to radiate up to the kinematical limit by the above `pTmaxMatch` choice, and no matrix-element corrections are available. Then, in many processes, the fall-off in pT will be too slow by one factor of pT^2. That is, while showers have an approximate dpT^2/pT^2 shape, often it should become more like dpT^2/pT^4 at pT values above the scale of the hard process. This argument is more obvious and relevant for ISR, where emissions could go the the kinematical limit, whereas they are constrained by the respective dipole mass for FSR. Nevertheless this matching option is offered for FSR to have a (semi-)symmetric description. Note that a dampening factor is applied to all dipoles in the final state of the hard process, which is somewhat different from the ISR implementation. `emissions go up to the kinematical limit, with no special dampening.` `emissions go up to the kinematical limit, but dampened by a factor k^2 Q^2_fac/(pT^2 + k^2 Q^2_fac), where Q_fac is the factorization scale and k is a multiplicative fudge factor stored in `pTdampFudge` below.` `emissions go up to the kinematical limit, but dampened by a factor k^2 Q^2_ren/(pT^2 + k^2 Q^2_ren), where Q_ren is the renormalization scale and k is a multiplicative fudge factor stored in `pTdampFudge` below.` `as option 1, but in addition to the standard requirements for dampening it is further necessary to have ar least two top or beyond-the-Standard-Model coloured particles in the final state. Examples include t tbar and squark gluino production.` `as option 2, but in addition to the standard requirements for dampening it is further necessary to have ar least two top or beyond-the-Standard-Model coloured particles in the final state. Examples include t tbar and squark gluino production.`

Note:

These options only apply to the hard interaction. Specifically, a "second hard" interaction would not be affected. Emissions off subsequent multiparton interactions are always constrained to be below the factorization scale of the process itself.
In cases 1 and 2 above, where a dampening is imposed at around the factorization or renormalization scale, respectively, this allows the pT scale of dampening of radiation by a half to be shifted by this factor relative to the default Q_fac or Q_ren. This number ought to be in the neighbourhood of unity, but variations away from this value could do better in some processes.

The amount of QCD radiation in the shower is determined by The alpha_strong value at scale M_Z^2. The default value corresponds to a crude tuning to LEP data, to be improved.

The actual value is then regulated by the running to the scale pT^2, at which the shower evaluates alpha_strong.

Order at which alpha_strong runs, `zeroth order, i.e. alpha_strong is kept fixed.` `first order, which is the normal value.` `second order. Since other parts of the code do not go to second order there is no strong reason to use this option, but there is also nothing wrong with it.`

The CMW rescaling of Lambda_QCD (see the section on StandardModelParameters) can be applied to the alpha_strong values used for timelike showers. Note that tunes using this option need lower values of alpha_strong(m_Z^2) than tunes that do not. `Do not apply the CMW rescaling.` `Apply the CMW rescaling, increasing Lambda_QCD for timelike showers by a factor roughly 1.6.`

QED radiation is regulated by the alpha_electromagnetic value at the pT^2 scale of a branching.

The running of alpha_em. `first-order running, constrained to agree with `StandardModel:alphaEMmZ` at the Z^0 mass.` `zeroth order, i.e. alpha_em is kept fixed at its value at vanishing momentum transfer.` `zeroth order, i.e. alpha_em is kept fixed, but at `StandardModel:alphaEMmZ`, i.e. its value at the Z^0 mass.`

The natural scale for couplings, and PDFs for dipoles stretching out to the beam remnants, is pT^2. To explore uncertainties it is possibly to vary around this value, however, in analogy with what can be done for hard processes. (Note that there is also an automated framework for shower uncertainties.)

The default pT^2 renormalization scale is multiplied by this prefactor. For QCD this is equivalent to a change of Lambda^2 in the opposite direction, i.e. to a change of alpha_strong(M_Z^2) (except that flavour thresholds remain at fixed scales).
The default pT^2 factorization scale is multiplied by this prefactor.

The rate of radiation if divergent in the pT → 0 limit. Here, however, perturbation theory is expected to break down. Therefore an effective pT_min cutoff parameter is introduced, below which no emissions are allowed. The cutoff may be different for QCD and QED radiation off quarks, and is mainly a technical parameter for QED radiation off leptons.

Parton shower cut-off pT for QCD emissions.
Parton shower cut-off pT for photon coupling to coloured particle.
Parton shower cut-off pT for pure QED branchings. Assumed smaller than (or equal to) `pTminChgQ`.

Shower branchings gamma → f fbar, where f is a quark or lepton, in part compete with the hard processes involving gamma^*/Z^0 production. In order to avoid overlap it makes sense to correlate the maximum gamma mass allowed in showers with the minimum gamma^*/Z^0 mass allowed in hard processes. In addition, the shower contribution only contains the pure gamma^* contribution, i.e. not the Z^0 part, so the mass spectrum above 50 GeV or so would not be well described.

Maximum invariant mass allowed for the created fermion pair in a gamma → f fbar branching in the shower.

### Interleaved evolution

Multiparton interactions (MPI) and initial-state showers (ISR) are always interleaved, as follows. Starting from the hard interaction, the complete event is constructed by a set of steps. In each step the pT scale of the previous step is used as starting scale for a downwards evolution. The MPI and ISR components each make their respective Monte Carlo choices for the next lower pT value. The one with larger pT is allowed to carry out its proposed action, thereby modifying the conditions for the next steps. This is relevant since the two components compete for the energy contained in the beam remnants: both an interaction and an emission take away some of the energy, leaving less for the future. The end result is a combined chain of decreasing pT values, where ones associated with new interactions and ones with new emissions are interleaved.

There is no corresponding requirement for final-state radiation (FSR) to be interleaved. Such an FSR emission does not compete directly for beam energy (but see below), and also can be viewed as occurring after the other two components in some kind of time sense. Interleaving is allowed, however, since it can be argued that a high-pT FSR occurs on shorter time scales than a low-pT MPI, say. Backwards evolution of ISR is also an example that physical time is not the only possible ordering principle, but that one can work with conditional probabilities: given the partonic picture at a specific pT resolution scale, what possibilities are open for a modified picture at a slightly lower pT scale, either by MPI, ISR or FSR? Complete interleaving of the three components also offers advantages if one aims at matching to higher-order matrix elements above some given scale.

If on, final-state emissions are interleaved in the same decreasing-pT chain as multiparton interactions and initial-state emissions. If off, final-state emissions are only addressed after the multiparton interactions and initial-state radiation have been considered.

As an aside, it should be noted that such interleaving does not affect showering in resonance decays, such as a Z^0. These decays are only introduced after the production process has been considered in full, and the subsequent FSR is carried out inside the resonance, with preserved resonance mass.

One aspect of FSR for a hard process in hadron collisions is that often colour dipoles are formed between a scattered parton and a beam remnant, or rather the hole left behind by an incoming partons. If such holes are allowed as dipole ends and take the recoil when the scattered parton undergoes a branching then this translates into the need to take some amount of remnant energy also in the case of FSR, i.e. the roles of ISR and FSR are not completely decoupled. The energy taken away is bookkept by increasing the x value assigned to the incoming scattering parton, and a reweighting factor x_new f(x_new, pT^2) / x_old f(x_old, pT^2) in the emission probability ensures that not unphysically large x_new values are reached. Usually such x changes are small, and they can be viewed as a higher-order effect beyond the accuracy of the leading-log initial-state showers.

This choice is not unique, however. As an alternative, if nothing else useful for cross-checks, one could imagine that the FSR is completely decoupled from the ISR and beam remnants.

If on, the final-state shower is allowed to borrow energy from the beam remnants as described above, thereby changing the mass of the scattering subsystem. If off, the partons in the scattering subsystem are constrained to borrow energy from each other, such that the total four-momentum of the system is preserved. This flag has no effect on resonance decays, where the shower always preserves the resonance mass, cf. the comment above about showers for resonances never being interleaved.
When beam recoil is allowed there is still some ambiguity how far into the beam end of the dipole that emission should be allowed. It is dampened in the beam region, but probably not enough. When on an additional suppression factor 4 pT2_hard / (4 pT2_hard + m2) is multiplied on to the emission probability. Here pT_hard is the transverse momentum of the radiating parton and m the off-shell mass it acquires by the branching, m2 = pT2/(z(1-z)). Note that m2 = 4 pT2_hard is the kinematical limit for a scattering at 90 degrees without beam recoil.

When there is no interleaving, a number of MPIs may have been generated before FSR is considered. In principle there could be colour correlations between the MPIs, such that a final-state colour of one MPI could be matched by the corresponding final-state anticolour of another MPI. These thereby would form a colour dipole, but one that does not come out from a common vertex, and therefore presumably could not radiate in full. Currently the standard procedure is to match colours between MPIs only after FSR, so MPI systems would radiate independently, with recoils taken by the beam remnant, where necessary. This could change, however, and the following switch would then regulate the choice of behaviour.

If on, and if interleaving is off, then dipoles are allowed to be formed between matching final-state colour-anticolour pairs also between two different MPIs. Else dipoles can normally only form inside the same MPI, and the could-have-been dipoles between different MPIs instead appear as dipoles stretched to the beam remnants. In either case a dipole can still form between two MPIs if a final-state colour cannot be matched inside the same MPI. This should normally not happen, except if rescattering is allowed, whereby two or more MPIs get interconnected.

### Global recoil

The final-state algorithm is based on dipole-style recoils, where one single parton takes the full recoil of a branching. This is unlike the initial-state algorithm, where the complete already-existing final state shares the recoil of each new emission. As an alternative, also the final-state algorithm contains an option where the recoil is shared between all partons in the final state. Thus the radiation pattern is unrelated to colour correlations. This is especially convenient for some matching algorithms, like MC@NLO, where a full analytic knowledge of the shower radiation pattern is needed to avoid double-counting. (The pT-ordered shower is described in Sjo05, and the corrections for massive radiator and recoiler in Nor01.)

Technically, the radiation pattern is most conveniently represented in the rest frame of the final state of the hard subprocess. Then, for each parton at a time, the rest of the final state can be viewed as a single effective parton. This "parton" has a fixed invariant mass during the emission process, and takes the recoil without any changed direction of motion. The momenta of the individual new recoilers are then obtained by a simple common boost of the original ones.

This alternative approach will miss out on the colour coherence phenomena. Specifically, with the whole subcollision mass as "dipole" mass, the phase space for subsequent emissions is larger than for the normal dipole algorithm. The phase space difference grows as more and more gluons are created, and thus leads to a way too steep multiplication of soft gluons. Therefore the main application is for the first one or few emissions of the shower, where a potential overestimate of the emission rate is to be corrected for anyway, by matching to the relevant matrix elements. Thereafter, subsequent emissions should be handled as before, i.e. with dipoles spanned between nearby partons. Furthermore, only the first (hardest) subcollision is handled with global recoils, since subsequent MPI's would not be subject to matrix element corrections anyway.

In order for the mid-shower switch from global to local recoils to work, colours are traced and bookkept just as for normal showers; it is only that this information is not used in those steps where a global recoil is requested. (Thus, e.g., a gluon is still bookkept as one colour and one anticolour dipole end, with half the charge each, but with global recoil those two ends radiate identically.)

Alternative approach as above, where all final-state particles share the recoil of an emission.
If off, then use the standard dipole-recoil approach.
If on, use the alternative global recoil, but only for the first interaction, and only while the number of particles in the final state is at most `TimeShower:nMaxGlobalRecoil` before the branching.
Represents the maximum number of particles in the final state for which the next final-state emission can be performed with the global recoil strategy. This number counts all particles, whether they are allowed to radiate or not, e.g. also Z^0. Also partons created by initial-state radiation emissions counts towards this sum, as part of the interleaved evolution. Without interleaved evolution this option would not make sense, since then a varying and large number of partons could already have been created by the initial-state radiation before the first final-state one, and then there is not likely to be any matrix elements available for matching.

Two variations of the scheme outlined above are also available, (motivated by comparative studies within aMC@NLO). These studies indicate that global recoils should be used as sparsely as possible, in order to retain desirable features of the radiation pattern produced with the local recoil prescription.

Choice which splittings are produced with the global recoil approach. `Global recoil mode as outlined above, i.e. using global recoils until the number of final state particles exceeds `TimeShower:nMaxGlobalRecoil`.` `Global recoil only for the first branching of final state legs that have an ancestor in the hard process, and if the maximal number of branchings generated according to the global recoil scheme (see `TimeShower:nMaxGlobalBranch` below) has not yet been reached.` `Global recoil only if the first branching in the whole evolution is a timelike splitting of a parton in an event with Born-like kinematics (i.e.\ an S-event). The impact of global recoils should be minimal in this case. This option is only sensible for interleaved evolution.`
The maximum number of splittings in the final state for which the next final-state emission can be performed with the global recoil strategy. This number has to be set if `TimeShower:globalRecoilMode = 1` or `TimeShower:globalRecoilMode = 2`
The number of partons for Born-like phase space points. This number needs to be set if a different treatment of S-events (with Born-like kinematics) and H-events (with real-emission kinematics) is desired. This number has to be set if `TimeShower:globalRecoilMode = 2`.
If on, limit the maximal pT produced in branchings in the global recoil scheme exactly as in the default (local) scheme. This means that the mass of the splitting dipole will set an upper bound for the pT of an emission. To be more explicit, this disallows emissions with pT larger than min{μstart 2, mD2/4}, with mD2 = (√ (pr +ps)2 -m0,s)2 - m0,r2 , where the shower starting scale is μstart (i.e. SCALUP when reading LHE files, and `Info.QFac()` otherwise), r the radiating parton, and s the recoiling particle that would have been used in the local recoil scheme. This option is only used if wimpy showers are enabled.

The global-recoil machinery does not work well with rescattering in the MPI machinery, since then the recoiling system is not uniquely defined. `MultipartonInteractions:allowRescatter = off` by default, so this is not a main issue. If both options are switched on, rescattering will only be allowed to kick in after the global recoil has ceased to be active, i.e. once the `nMaxGlobalRecoil` limit has been exceeded. This should not be a major conflict, since rescattering is mainly of interest at later stages of the downwards pT evolution.

Further, it is strongly recommended to set `TimeShower:MEcorrections = off` (not default!), i.e. not to correct the emission probability to the internal matrix elements. The internal ME options do not cover any cases relevant for a multibody recoiler anyway, so no guarantees are given what prescription would come to be used. Instead, without ME corrections, a process-independent emission rate is obtained, and user hooks can provide the desired process-specific rejection factors.

### Radiation off octet onium states

In the current implementation, charmonium and bottomonium production can proceed either through colour singlet or colour octet mechanisms, both of them implemented in terms of 2 → 2 hard processes such as g g → (onium) g. In the former case the state does not radiate and the onium therefore is produced in isolation, up to normal underlying-event activity. In the latter case the situation is not so clear, but it is sensible to assume that a shower can evolve. (Assuming, of course, that the transverse momentum of the onium state is sufficiently high that radiation is of relevance.)

There could be two parts to such a shower. Firstly a gluon (or even a quark, though less likely) produced in a hard 2 → 2 process can undergo showering into many gluons, whereof one branches into the heavy-quark pair. Secondly, once the pair has been produced, each quark can radiate further gluons. This latter kind of emission could easily break up a semibound quark pair, but might also create a new semibound state where before an unbound pair existed, and to some approximation these two effects should balance in the onium production rate. The showering "off an onium state" as implemented here therefore should not be viewed as an accurate description of the emission history step by step, but rather as an effective approach to ensure that the octet onium produced "in the hard process" is embedded in a realistic amount of jet activity. Of course both the isolated singlet and embedded octet are likely to be extremes, but hopefully the mix of the two will strike a reasonable balance. However, it is possible that some part of the octet production occurs in channels where it should not be accompanied by (hard) radiation. Therefore reducing the fraction of octet onium states allowed to radiate is a valid variation to explore uncertainties.

If an octet onium state is chosen to radiate, the simulation of branchings is based on the assumption that the full radiation is provided by an incoherent sum of radiation off the quark and off the antiquark of the onium state. Thus the splitting kernel is taken to be the normal q → q g one, multiplied by a factor of two. Obviously this is a simplification of a more complex picture, averaging over factors pulling in different directions. Firstly, radiation off a gluon ought to be enhanced by a factor 9/4 relative to a quark rather than the 2 now used, but this is a minor difference. Secondly, our use of the q → q g branching kernel is roughly equivalent to always following the harder gluon in a g → g g branching. This could give us a bias towards producing too hard onia. A soft gluon would have little phase space to branch into a heavy-quark pair however, so the bias may not be as big as it would seem at first glance. Thirdly, once the gluon has branched into a quark pair, each quark carries roughly only half of the onium energy. The maximum energy per emitted gluon should then be roughly half the onium energy rather than the full, as it is now. Thereby the energy of radiated gluons is exaggerated, i.e. onia become too soft. So the second and the third points tend to cancel each other.

Finally, note that the lower cutoff scale of the shower evolution depends on the onium mass rather than on the quark mass, as it should be. Gluons below the octet-onium scale should only be part of the octet-to-singlet transition.

Allow colour-octet charmonium and bottomonium states to radiate gluons. 0 means that no octet-onium states radiate, 1 that all do, with possibility to interpolate between these two extremes.
The colour factor used used in the splitting kernel for those octet onium states that are allowed to radiate, normalized to the q → q g splitting kernel. Thus the default corresponds to twice the radiation off a quark. The physically preferred range would be between 1 and 9/4.

### Weak showers

The emission of weak gauge bosons is an integrated part of the initial- and final-state radiation, see Weak Showers. The following settings are those specifically related to the final-state weak radiation, while common settings are found in the Weak Showers description.

Allow a weak shower, yes or no.
Determine which branchings are allowed. `both W^+- and Z^0 branchings.` `only W^+- branchings.` `only Z^0 branchings.`
Parton shower cut-off pT for weak branchings.

### Further variables

There are several possibilities you can use to switch on or off selected branching types in the shower, or in other respects simplify the shower. These should normally not be touched. Their main function is for cross-checks.

Allow a QCD shower, i.e. branchings q → q g, g → g g and g → q qbar; on/off = true/false.
Number of allowed quark flavours in g → q qbar branchings (phase space permitting). A change to 4 would exclude g → b bbar, etc.
Different options to assign kinematics distributions and weights for g → q qbar branchings, notably for charm and bottom quarks. These options also have the corresponding effect on gamma → f fbar branchings. The rationale for the options is described in this note.
Notation: r_q = m_q^2/m_qq^2, beta = sqrt(1 - 4r_q), with m_q the quark mass and m_qq the q qbar pair invariant mass. The scale factor k is described below, `TimeShower:scaleGluonToQuark`. `same splitting kernel (1/2) (z^2 + (1-z)^2) for massive as massless quarks, only with an extra beta phase space factor.` `a splitting kernel (beta/2) (z^2 + (1-z)^2 + 8r_q z(1-z)).` `a splitting kernel z^2 + (1-z)^2 + 8r_q z(1-z), normalized so that the z-integrated rate is (beta/3) (1 + r/2).` `same as 3, but additionally a suppression factor (1 - m_qq^2/m_dipole^2)^3, which reduces the rate of high-mass q qbar pairs.` `same as 1, but reweighted to an alpha_s(k m_qq^2) rather than the normal alpha_s(pT^2).` `same as 2, but reweighted to an alpha_s(k m_qq^2) rather than the normal alpha_s(pT^2).` `same as 3, but reweighted to an alpha_s(k m_qq^2) rather than the normal alpha_s(pT^2).` `same as 4, but reweighted to an alpha_s(k m_qq^2) rather than the normal alpha_s(pT^2).`
Extra scale parameter k for `TimeShower:weightGluonToQuark` options 5 - 8. Comes on top of `TimeShower:renormMultFac`, which affects alpha_s(pT^2) alike.
For topologies where a gluon recoils against a massive quark (or another massive coloured particle) there are no suitable ME corrections implemented into PYTHIA. When the dipole radiation pattern is split into two ends, with a smooth transition between the two, this means that the gluon end can radiate into the quark hemisphere as if the quark were massless. The "dead cone" effect, that radiation collinear with a massive quark is strongly suppressed, thereby is not fully respected. (Unlike radiation from the quark end itself, where mass effects are included.) With this switch on, a further suppression is therefore introduced for g → g g branchings, derived as the massive/massless ratio of the eikonal expression for dipole radiation, which kills radiation collinear with the quark. The g → q qbar branchings currently are not affected; the absence of a soft singularity implies that there is hardly any radiation into the recoiler hemisphere anyway.
Allow quarks to radiate photons, i.e. branchings q → q gamma; on/off = true/false.
Allow leptons to radiate photons, i.e. branchings l → l gamma; on/off = true/false.
Allow charged resonances to radiate photons, i.e. branchings q∼ → q∼ gamma; on/off = true/false. This will also allow the W boson to radiate.
Allow photons to branch into lepton or quark pairs, i.e. branchings gamma → l+ l- and gamma → q qbar; on/off = true/false.
Number of allowed quark flavours in gamma → q qbar branchings (phase space permitting). A change to 4 would exclude g → b bbar, etc.
Number of allowed lepton flavours in gamma → l+ l- branchings (phase space permitting). A change to 2 would exclude gamma → tau+ tau-, and a change to 1 also gamma → mu+ mu-.
Use of matrix element corrections where available; on/off = true/false.
Use matrix element corrections also for 1 → n and 2 → n processes where no matrix elements are encoded, by an attempt to match on to one of the 1 → 2 processes that are implemented. This should at least provide relevant mass dampening for massive radiators and recoilers. Only has a meaning if `MEcorrections` above is switched on.
Use of matrix element corrections also after the first emission, for dipole ends of the same system that did not yet radiate. Only has a meaning if `MEcorrections` above is switched on. Switching off this option currently does not take effect for a few rare types of secondary branchings, where ME corrections play a central role.
Azimuthal asymmetry induced by gluon polarization; on/off = true/false.
Extend the above azimuthal asymmetry (if on) also back to gluons produced in the hard process itself, where feasible; on/off = true/false.
In the decays of coloured resonances, say t → b W, it is not possible to set up dipoles with matched colours. Originally the b radiator therefore has W as recoiler, and that choice is unique. Once a gluon has been radiated, however, it is possible either to have the unmatched colour (inherited by the gluon) still recoiling against the W (`off`), or else let it recoil against the b also for this dipole (`on`). Before version 8.160 the former was the only possibility, which could give unphysical radiation patterns. It is kept as an option to check backwards compatibility. The same issue exists for QED radiation, but obviously is less significant. Consider the example W → e nu, where originally the nu takes the recoil. In the old (`off`) scheme the nu would remain recoiler, while in the new (`on`) instead each newly emitted photon becomes the new recoiler.
Allow the possibility to use a fixed factorization scale, set by the `parm` below. This option is unphysical and only intended for toy-model and debug studies.
The fixed factorization scale, in GeV, that would be used in the evaluation of parton densities if the `flag` above is on.